Switching DC/DC converters commonly require sensing inductor current for current control or monitoring purposes, such as time averaged current control, current sharing control, output current monitoring and so on. For example, a conventional step-down converter, or called buck converter, which steps down voltage (while stepping up current) from its input to its output (or load), includes a digitally-controlled transistor, a diode, an inductor, a capacitor bridged the output of the buck converter. For the sensing of the inductor current, a current sensing shunt including amplifier and low pass filter circuitry can be employed in parallel with a current sensing resistor connected between the diode and capacitor of the step-down converter. However, power dissipation of the whole circuit will be inevitably increased due to large current flowing through the current sensing resistor. Alternatively, a Hall effect sensor may be used. However, the Hall effect sensor has the problem of bandwidth limitation and is not suitable for sensing switching signals.
In another example, a current transformer can be applied to the above step-down converter instead of the current sensing resistor and current sensing shunt, without the bandwidth limitation and power loss increase for the other approaches. The current transformer, however, is useful only if AC current is to be measured or the sensed current is in pulse form because DC component may saturate the current transformer and thus measurement distortion occurs. In addition, the current sensing should be deliberately conducted in the discontinuous conduction mode of the step-down converter since the inductor current in the discontinuous conduction mode is asymmetrical, as compared to the symmetrical AC component of continuous conduction mode.
Thus, it is desirable to have improvements on the conventional current sensing approach in order to sense an inductor current for a DC/DC converter.